Method and system for load sharing between a plurality of cells in a radio network system

ABSTRACT

The present invention relates to a method and a system for load sharing between a plurality of cells in a radio network system, wherein each of said cells is serving a number of mobile devices. For handover of a mobile device from a currently serving source cell to a target cell, a priorization among cells adjacent to the source cell is carried out by calculating the power budgets between the currently serving source cell on the one hand and said adjacent cells on the other hand, and the adjacent cell having the highest priority is selected as target cell.

FIELD OF THE INVENTION

The present invention relates to a method and a system for load sharingbetween a plurality of cells in a radio network system, wherein each ofsaid cells is serving a number of mobile devices.

BACKGROUND OF THE INVENTION

Traffic management in mobile cellular networks comprises a set offeatures whose final purpose is the redistribution of the mobile devicesin the cells so that the probability of congestion and blocking isreduced and, thus, capacity is increased and quality is improved.

In current systems, the design is based on the hypothesis of a uniformdistribution of served mobile devices. However, such assumption may befar from reality. Namely, the flexibility resulting from the existinglarge set of parameters included in the different algorithms related totraffic distribution can not be fully seized because of its complexity.The large set of available parameters makes the detailed planningprocess on a cell-by-cell basis a time-consuming work.

As a consequence, operators fix parameters to a common set of defaultvalues shared between the cells even if the performance and capacity isnot achieved in an optimum way. Moreover, a few operators extend theparameter optimisation by classifying the cells in accordance withcertain scenarios like rural, urban, tunnel, indoors etc. and/or inaccordance with the layer/band used (like Macro900/1800, Micro900/1800,Pico1800, Motorway900). So, the cells are divided into scenario groupsor layer/band groups, and common default parameter values are sharedwhich, however, are not optimum.

In those cases where existing features for traffic balance andcongestion relief are difficult to optimise, just a few parameters aretaken into account for optimisation which requires so-called fieldtrials. During the tuning process, conclusions from parameter changesare difficult to derive, and final settings are nearly always on thesafe side with its limited results. Moreover, such trials are normallyfocused on global parameters of features under study, and parameteroptimisation of adjacent cells is hardly ever done. So, differencesbetween adjacent cells are rarely considered due to a high effortrequired. Therefore, the potential of so-called adjacency parameters isnot fully exploited.

A final limited parameter tuning based on cell/area level performanceindicators is normally carried out only over those cells whereperformance problems are existing.

Even if an optimum value were reached by means of the above-mentionedtrials, changes in traffic or environment conditions, like theinstallation of new cells, changes of interference level by frequencyre-planning etc., would force a further re-tuning process of theparameter base, where no automatic reactive process is currently in use.Such a situation could be analysed as a result of slow trends, like thechange of the number of user registrations, or fast changes, e.g. of thenumber of connections, during a short time period, like an hour or aday.

One of the critical causes of network variations is interference.Differences in propagation conditions between cells or changes in thefrequency plan will produce variations in time or space. Adaptation tothis variations would increase network performance, but would alsorequire a very high tuning effort.

U.S. Pat. No. 5,241,685 A discloses a load sharing control for a mobilecellular radio system so as to achieve a load sharing between a firstcell and a second cell adjacent to the first cell where each cell isserving a number of mobile devices. The first cell has a predeterminedentering threshold which is a function of the received signal strengthfor mobile devices entering this cell from the second adjacent cell bymeans of handoff. A certain occupancy level indicates the occupiedchannels in relation to the available channels in the cell. Forhandover, the occupancy level of the first and second cells aredetermined, and it is further determined whether the second cell has alower occupancy level than the first cell. Then, an entering thresholdlevel for the second cell is determined which is a function of thereceived signal strengths for the mobile devises in the first cell aboutto enter the second cell. The entering threshold for the second cell isdecreased if the occupancy level of the second cell is found to be lowerthan the occupancy level of the first cell, whereby the border betweenthe first and second cells is dynamically changed. So, in this knownsystem, the redistribution of the users for congestion relief is usuallyachieved by shrinking loaded cells through temporarily reduced marginsfor handovers between adjacent cells.

However, this prior art system has the disadvantages of reduced qualityin traffic receiving cells and reduced overlapping in activation periodswherein the latter results in ping-pong problems.

SUMMARY OF THE INVENTION

It is an object of the present invention to improve the load sharingbetween a plurality of cells in a radio network system, in particular bytaking congestion relief with quality constraints into account.

In order to overcome the above and further objects, according to a firstaspect of the present invention, there is provided a method for loadsharing between a plurality of cells in a radio network system, whereineach of said cells is serving a number of mobile devices, characterizedin that, for handover of a mobile device from a currently serving sourcecell to a target cell, a prioritisation among cells adjacent to thesource cell is carried out by calculating the associated power budgetsbetween the currently serving source cell on the one hand and saidadjacent cells on the other hand, and the adjacent cell having thehighest priority is selected as target cell.

According to a second aspect of the present invention, there is provideda system for load sharing between a plurality of cells in a radionetwork system, wherein each of said cells is serving a number of mobiledevices, characterized by means for calculating the power budgetsbetween a source cell which is currently serving a mobile device to behanded over to a target cell on the one hand and cells adjacent saidsource cell on the other hand, means for priorisation among saidadjacent cells in accordance with the result calculated by thecalculating means, and means for selecting the adjacent cell having thehighest priority as target cell.

So, in accordance with the present invention, once a handover istriggered, candidate cells are prioritised by calculation of theirassociated power budget margins.

An advantage of the present invention is that traffic and environmentchanges are tracked by means of automatic parameter auto-tuning in orderto achieve the best performance without user interaction. As aconsequence, less parameters are required to be adjusted. Further, themargins can be smoothly changed so that there is a smooth behaviour ofthe load sharing control. Moreover, the present invention provides for abetter performance since quality and overlapping constraints areincluded together with capacity constraints so as to add moreinformation in the control of the operational area of the cellsconcerned. In particular, better control of the operational area of thecells is achieved as adjacent parameters control the shape of the cell,and adaptation to cell differences can be easily carried out.

In particular, the final prioritising criterion is the received downlinkpower difference from a beacon channel.

In a preferred embodiment of the invention, the power budget PBGT (n) ofthe nth cell (n=1, 2, . . . , N; N>1) is calculated by using theequationPBGT(n)=RxLev _(adj) −RxLev _(serv) −HoMarginPBGT _(serv)(adj)  (1)wherein RxLev_(adj) is the receiving level of the adjacent cell,RxLev_(serv) is the receiving level of the current serving source celland HoMarginPBGT_(serv)(adj) is a handover margin parameter.

In a preferred embodiment, the priorisation is carried out by takinginto account both load and quality performance. So, displacement ofoverlapping area between adjacent cells may be controlled by both loadand quality indicators, while overlapping may still be assured if thesum of margins is maintained unchanged. When using the above equation(1), the handover margin parameter HoMarginPBGT_(serv)(adj) should be afunction of both load and quality performance.

Usually, when the currently serving source cell is congested, itsoperational size is reduced.

The shrinking of the operational size of the cell concerned can beperformed by means of relaxing handover conditions to the neighbouringcells. This action can be translated into a decrement of theHoMarginPBGTserving→adj parameters to adjacent cells. In the inversedirection of the ‘adjacency’, opposite action must be carried out forthe HoMarginPBGT_(adj←serving) parameters of the adjacent cells, namelysuch parameters are incremented, so as to maintain constant overlappingareas between involved cells. Due to the fact that constant overlappingbetween involved cells may be desirable in order to avoid ping-pongeffects, simultaneous adjustment of symmetric parameters should becarried out to modify by the same amount, of course to oppositedirection. In this way, saturation in the adjacency only in onedirection, caused by reaching operator constraints in parameter values,may easily be considered.

The present invention can be implemented in any kind of cellular mobilenetwork systems like i.e. GSM or UMTS, or in internet protocol radioaccess networks. It should be also noted that the invention can beimplemented in a multiradio environments where many different airinterference protocols are used.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the present invention will be described in greaterdetail based on a preferred embodiment with reference to theaccompanying drawing Figures in which

FIG. 1 is a schematical diagram showing a process for the finalparameter setting. The proposed final values (i.e.:output) for theadjacency automated margins are derived from congestion and qualityindicators of serving and adjacency target cells. From differences ofboth indicators, a conclusion for changes in involved parameters isdrawn;

FIG. 2 shows an example of a table showing percentage of samples inpredefined defined downlink quality (columns) and received levelintervals (rows);

FIG. 3 shows a three-dimensional plot indicating a cumulative densityfunction extracted based on samples from the previous table, and agraphical representation of how a conclusion for the minimum requiredlevel is drawn from the quality requirements;

FIG. 4 shows a graph indicating the curve which represents theintersection between surfaces in the previous figure, where it isclearly seen the relationship between required quality and minimumrequired level;

FIG. 5. shows a graph indicating the differences between adjacent cellsin terms of, required level for ascertain quality (i.e.:Quality-to-Minimum Level mapping). Averaging over quality values morelikely to happen is carried out to calculate a final unique globalindicator of the difference; and

FIG. 6 shows a block diagram of a preferred embodiment of a controlstructure for power budget margin automation in accordance with apreferred embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An automatic tuning process for handover margins used in a target-cellprioritisation algorithm is here described.

In radio access networks, candidate cells are prioritised throughcalculation of their association power budget. Discarding minorcompensation terms, the evaluation of this indicator uses the equationPBGT(n)=RxLev _(adj) −RxLev _(serv) −HoMarginPBGT _(serv)(adj)  (1)wherein RxLev_(adj) is the receiving power level of the nth adjacentcell, RxLev_(serv) is the receiving power level of the currently servingsource cell, and HoMarginPBGT_(serv)(adj) is a handover marginparameter.

The remaining adjacent handover margin parameterHoMarginPBGT_(serv)(adj) is normally used to assure hysteresis (i.e.overlapping) region to avoid instabilities from ping-pong problems. Inbest cases, some operators bias the priority evaluation to cellsbelonging to capacity layers (i.e. micro and pico cells). In somesituations, scenario considerations (e.g. average interference in tightfrequency reuse, propagation severity differences in indoor-outdoorenvironments/900–1800 MHz bands) may also be included by experiencedoperators, but no differences between cells in the same class or timeperiods are accounted for.

A control method for this handover margin parameter is provided in orderto achieve prioritisation of adjacent cells taking into accountdifferences in the signal level requirements. Thus, displacement ofoverlapping area between adjacent cells is controlled by both load andquality indicators, while overlapping may still be assured if the sum ofmargins is maintained unaltered. Doing so, the space of the cell iscontrolled.

A key factor resides on the ability to define a mapping function betweenreceived downlink level from BCCH channel (broadcast control channel)and predicted quality of assigned channel. This function will make useof mobile measurement reports extracted dynamically from the network,easily differentiating among cells or time. In this way, the automationalgorithm will cope with scenario differences (i.e. interference,propagation severity) due to traffic trend changes both in space andtime.

FIG. 1 schematically shows a tuning process for providing minimum levelthresholds from measurement reports wherein in particular shown is theflow of information from the network to the final parameter setting.Collection of mobile measurements from users in connected mode isundertaken in every cell. By means of further processing of these rawcounters, the probability density function PDF=f_(cell)(Received Level,Perceived Channel Quality) construction is straightforward.

A tabular representation is depicted in FIG. 2 showing an example of atable structure of RxLev/RxQual counters from Rx-level statistics (TRXlevel, downlink),

wherein RxLev is the receiving level and RxQual is the receiving qualityand the Rx-Level is the receiving level.

From this piece of information, the probability of perceiving at least acertain connection quality may be extracted for every measured receivedlevel. This three-dimensional function is depicted in FIG. 3 showing athree-dimensional plot of cumulative RxQual distribution from Rx-Levelstatistics on the basis of samples extracted from real data. So, thisfigure shows the appearance of cumulative quality distribution (i.e.probability of getting quality better than a certain value), for everylevel band. In the example, upper limits for 1,2,3,4,5,6 level bands are−100, −95, −90, −80, −70 and 47 dBm, respectively. It is worthwhile tonote that they were extracted from a real network management systemdatabase. The intersection between Confidence and RxLevel-RxQualthree-dimensional planes is the aimed relationship Quality-to-Level.

Once this Level-to-Quality relationship has been constructed, the nextsteps will aim at building the inverse Quality-to-Level function.

First, the confidence of the mapping process must be decided. Themeaning of this internal parameter of the algorithm relates to thelikelihood of the decision that a certain level is enough to get apredefined quality. Its graph is a plane whose intersection with thethree-dimensional probability function defines a unique curve relatingtarget connection quality to minimum required level (i.e.:Quality-to-Level relationship), represented in FIG. 4.

From this relationship, the comparison between cells is ratherstraight-forward, which is shown in FIG. 5. Those cells that require ahigher level to fulfill quality targets have their priority reduced infavour of those less power greedy. The adjacency handover margin shouldcorrespond to level differences between adjacent cells (equallydistributed through both margins in the adjacency, still maintaining aconstant residual term for overlapping).

The only remaining issue is the quality target which is used for levelcomparison purposes, as the quality cross point for handover is notknown. Averaging of level values through quality values where handovermay occur (i.e.: from Q0 to Quality Handover Threshold) must be done toget a unique value which reflects the difference in level requirementsin the adjacency.

In FIG. 6, the basic structure of the proposed control module applied inadjacency basis for the parameter margin parameterHoMarginPBGTserving→adj and HoMarginPBGT_(adj→serving) is presented.

In a first step, formulas are applied to raw counters in order toextract meaningful quality and congestion performance indicators fromsource (S) and target (T) cells. Either directly defining a costfunction, or just defining thresholds for acceptable and non-acceptablesituations (and building it in intervals with different slopes),operator controls the mapping function from raw counters to quality andcapacity problem indicators. A different mapping cost function is usedto extract the problem indicators related to quality and congestion,while they can be possibly shared between cells.

The selected controller follows an incremental structure, proposing theincrement/decrement from the previous values. Basically, when a cell iscongested, shrinking its operational size by means of relaxing handoverconditions to neighbouring cells can be performed for traffic balancingbetween cells. This action is translated into a decrement inHoMarginPBGT_(serving→adj) parameters to adjacent cells. In the inversedirection of the adjacency, opposite action must be carried out forHoMarginPBGT_(adj→serving) parameters to maintain constant overlappingbetween cells.

Due to the fact that constant overlapping between involved cells may bedesirable in order to avoid ping-pong effects, simultaneous adjustmentof symmetric parameters must be carried out to modify by the same amount(of course in opposite directions). In this way, saturation in theadjacency only in one direction, caused by reaching operator constraintsin parameter values, may easily be considered.

Being balance in problems (i.e. costs) between cells the last purpose ofthe automation action, the cost difference output controls the directionand magnitude of changes. This error signal related to problem indicatordifferences is to be minimised, as in most control systems. Theequilibrium point will be reached when this cost error between cells iszero (not necessarily individual costs). As it may be seen from a closeranalysis, this structure has a variable step size, proportional todeviation from stability final point.

In situations where trade-off is achieved (i.e.: serving cell iscongested and adjacent cell has bad quality), cost terms of oppositesign compensate, whenever cost values for threshold crossing areconsistent (i.e.: equal problem indicator for border problem). A finalequilibrium point (i.e. cost difference equal to 0.0) can be biased byweighting problem indicator difference terms with two priority factors,taking into account operator preferences in term of quality or capacitypriorities.

Calculated deviation from balance is adjusted by means of step control,selecting the aggressiveness of the tuning process, and thus influencingon the final trade-off between speed/stability of the convergenceprocess to the final equilibrium solution. A subsystem in a higher levelin the proposed hierarchical control architecture may govern the speed,based on instability detection (i.e.: oscillation in parameter values)or slow convergence.

Once deviation from current values is proposed, final checks must bedone against maximum and minimum limits constrained by the operator.

Although the invention is described above with reference to an exampleshown in the attached drawings, it is apparent that the invention is notrestricted to it, but can vary in many ways within the scope disclosedin the attached claims.

1. A method for load sharing between a plurality of cells in a radionetwork system, wherein each of said cells is serving a number of mobiledevices, comprising the steps of, for handover of a mobile device from acurrently serving source cell to a target cell, carrying out aprioritization among cells adjacent to the source cell by calculatingthe power budgets between the currently serving source cell on the onehand and said adjacent cells on the other hand, and selecting theadjacent cell having the highest priority as target cell, wherein ahandover margin parameter used in said power budget calculation is afunction of both load and quality performance, and, when the currentlyserving source cell is congested, its operational size is reduced whilekeeping the overlapping area between the involved cells substantiallyconstant.
 2. The method according to claim 1, wherein the finalprioritizing criterion is a received downlink power difference from abeacon channel.
 3. The method according to claim 1, wherein the powerbudget PBGT(n) of the nth cell (n=1, 2, 3 . . . , N; N>1) is calculatedby using the equationPBGT(n)=RxLev _(adj) −RxLev _(serv) −HoMarginPBGT _(serv)(adj), whereinRxLev_(adj)is the receiving level of the nth adjacent cell, RxLev_(serv)is the receiving level of the currently serving source cell,HoMarginPBGT_(serv)(adj) is said handover margin parameter.
 4. Themethod according to claim 3, wherein HoMarginPBGT_(serv)(adj) is afunction of a predetermined scenario.
 5. The method according to claim3, wherein HoMarginPBGT_(serv)(adj) is a function of a predeterminedhystereses.
 6. The method according to claim 3, wherein a handovermargin parameter HoMarginPBGT_(serving→adj) of the currently servingsource cell with respect to an adjacent cell is decremented.
 7. Themethod according to claim 6, wherein a handover margin parameterHoMarginPBGT_(adj→serving) of an adjacent cell with respect to thecurrently serving source cell is incremented.
 8. A system for loadsharing between a plurality of cells in a radio network, said systemcomprising: means for calculating the power budgets between a sourcecell which is currently serving a mobile device to be handed over to atarget cell on the one hand and cells adjacent to said currently servingsource cell on the other hand, said calculating means is adapted to usea handover margin parameter in said power budget calculation, saidhandover margin parameter being a function of both load and qualityperformance, means for priorization among said adjacent cells inaccordance with the result calculated by the calculating means, meansfor selecting the adjacent cell having the highest priority as targetcell, and means for reducing the operational size of the currentlyserving source cell in case of congestion, while keeping the overlappingarea between the involved cells substantially constant.
 9. The systemaccording to claim 8, wherein said priorization means uses a receivingdownlink power difference from a beacon channel as a final prioritizingcriterion.
 10. The system according to claim 8, wherein the calculatingmeans calculates the power budget PBGT(n) of the nth cell (n=1, 2, 3, .. . , N; N>1) by using the equationPBGT(n)=RxLev _(adj) −RxLev _(serv) −HoMarginPGBT _(serv)(adj), whereinRxLev_(adj) is the receiving level of the nth adjacent cell,RxLev_(serv) is the receiving level of currently serving source cell,and HoMarginPGBT_(serv)(adj) is said margin parameter.
 11. The systemaccording to claim 10, wherein HoMarginPGBT_(serv)(adj) is a function ofa predetermined scenario.
 12. The system according to claim 10, whereinHoMarginPGBT_(serv)(adj) is a function of a predetermined hystereses.13. The system according to claim 10, further comprising means fordecrementing a handover margin parameter HoMarginPGBT_(serving→adj) ofthe currently serving source cell with respect to an adjacent cell. 14.The system according to claim 13, further comprising means forincrementing a handover margin parameter HoMarginPGBT_(adj→serving) ofan adjacent cell with respect to the currently serving source cell.